1. Field of the Invention
The present invention relates generally to processes for manufacturing holographic elements and for transferring the same from an original substrate to another. More specifically, the present invention relates to a process for completely fabricating a holographic element on a highly stable substrate, such as glass, which allows transference of the same to a second substrate without appreciable damage to the holographic element. An improved moisture barrier for the holographic element is also disclosed.
2. Description of Related Art
Holograms are in wide use today. Of particular interest to Applicant is the use of holograms in automotive products, such as head-up displays and plastic visors.
Manufacturing techniques typically include coating a substrate with a light sensitive material (typically a dichromated gelatin), exposing that coated material to a controlled laser interference pattern containing the desired holographic image, processing the exposed coating, baking the processed coating to fine tune the wavelength of reflected light, (i.e. "baking to wavelength") and sealing the processed coating from adverse environmental elements.
Typically, the substrate is a thick, hard and flat material such as glass. After the hologram is recorded and sealed, the glass is cut and/or ground to meet thickness, weight and size requirements.
This typical procedure, however, is relatively expensive and is often accompanied by a high loss rate. In addition, it is difficult to produce thicknesses less than about 0.06 inch. Further, the process is difficult to adapt to curved surfaces.
Another technique is to use a thin film as the substrate. Unfortunately, this suffers from stability problems. Although clamping the thin film between glass plates to overcome this stability problem has been tried, the clamping often causes stress, resulting in optical imperfections. In addition, the film sometimes warps during the baking to wavelength step. In order to provide needed stability, moreover, the film often has to be so thick that it does not adapt well to double-curved surfaces such as the automobile window.
Another technique is to coat a traditional glass substrate with a poor mold release before applying the gelatin on which the holographic image is recorded. Thereafter, the gelatin is peeled from the glass substrate and deposited upon the final substrate.
One problem with use of a poor mold release, however, is that it does not hold up well. As a result, the gelatin must generally be transferred to a sheet of polystyrene before the wet processing step. Because the polystyrene absorbs water, however, a partial loss in hologram efficiency often results. During the baking to wavelength step, moreover, the gelatin is often subject to shrinkage, delamination and/or tearing because of difficulties in adhering the gelatin to the polystyrene layer.
Another technique is to start out by affixing the gelatin to the final substrate material, typically a piece of curved or bulky glass, acrylic or other transparent material. Because of the generally large size and/or unsuitability to the recording process, however, these are often difficult to coat, expose and/or process.
There therefore has been a continuing need for a method of producing holograms which has the stability of holograms recorded on glass substrates, but the flexibility of holograms produced on thin film-based substrates. There has also been a continuing need for holograms having superior moisture barrier properties so as to maximize use efficiency.